Simply put, Telnet is a program that allows you log on to a Unix-based computer. If you have an account on a Telnet server, you can access its resources (such as a directory of your files, e-mail, ftp, etc).
What is telnet?
Telnet and its close cousins rlogin and tn3270 are methods of connecting to a remote computer over the Internet that let you use programs and data just as if you were using the computer locally. Do not confuse telnet with Telenet, the old name for Sprintnet.
Telnet is a text-only protocol. At one time it was one of the most common ways to connect to other sites. Now with the growth of the web, it is hardly used. There are still a few sites and resources that can only be reached through telnet.
A telnet URL takes this form: telnet://some.address:port-number
telnet some.address port-number
Telnet is unsecure because it is unencrypted ,if possible you should use SSH instead.
Standard Telnet port is 23